1. Field of the Invention
The present invention relates to a method and apparatus for conveying or transporting a workpiece such as a semiconductor wafer, glass substrate or liquid crystal panel, and more particularly to a method and apparatus for conveying a workpiece between processing apparatuses when the workpiece is processed in a plurality of processing apparatuses.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus, and such a process is called Chemical Mechanical Polishing (CMP) in which the semiconductor wafers are chemically and mechanically polished while supplying an abrasive liquid comprising abrasive grains and chemical solution such as alkaline solution.
When semiconductor wafers are processed, small particles such as particles of semiconductor material, dust particles, crystalline protrusive particles, or the like often tend to be attached to the semiconductor wafers being processed. If a particle greater than the distance between interconnections exists on a semiconductor substrate, then the particle will short-circuit interconnections on the semiconductor substrate. Therefore, any undesirable particles on a semiconductor substrate have to be sufficiently smaller than the distance between interconnections on the semiconductor substrate. Such a problem and a requirement hold true for the processing of other workpieces including a glass substrate to be used as a mask, a liquid crystal display, and so on. To meet the above requirement, there have been practiced some cleaning procedures for removing fine particles or submicron particles from semiconductor wafers.
In this manner, the semiconductor wafers and the like are subjected to a plurality of processes. For example, after polishing, the semiconductor wafers are cleaned one by one and stored in a predetermined location. As a method of physically cleaning semiconductor wafers one by one, there have been scrubbing cleaning in which a surface of a semiconductor wafer is scrubbed by a brush of nylon or mohair or a sponge of polyvinyl alcohol (PVA), ultrasonic jet cleaning in which an ultrasonic jet is applied to the surface of the semiconductor wafer, and high pressure water cleaning in which high pressure water is applied to the surface of the semiconductor wafer. In cleaning the semiconductor wafer after polishing, a plurality of cleaning processes using the above two or three methods are applied to the semiconductor wafer. Therefore, it is necessary to convey or transfer the semiconductor wafer between cleaning processes. In this case, the semiconductor wafer is conveyed between cleaning processes in such a state that it is kept wet with liquid. This is because if the semiconductor wafer is not in a wet state before completing the cleaning processes, dust may be attached to the surface of the semiconductor wafer, and if the semiconductor wafer is half-dried, water marks may be formed on the surface of the semiconductor wafer. In particular, if the semiconductor wafer which has been subjected to the CMP process is once dried, the slurry attached to the polished surface of the semiconductor wafer is solidified thereon and is difficult to be removed therefrom, and hence the semiconductor wafer is kept wet with cleaning liquid between cleaning processes.
However, if the wet semiconductor wafer is conveyed at a high speed, as is the case in a recent high through-put-type apparatus, liquid on the surface of the semiconductor wafer may drop from the semiconductor wafer. The dropped liquid, when it is dried and solids contained in the liquid rise up, becomes a source of particles. Further, in the case where different cleaning liquids are used in the cleaning processes, an unexpected chemical reaction may occur upon contact of the different cleaning liquids.